1. Field of the Invention
Embodiments disclosed herein relate generally to shale shakers and screens for shale shakers. Specifically, embodiments disclosed herein relate to a shale shaker configured to engage a wedge-like screen frame.
2. Background
Oilfield drilling fluid, often called “mud,” serves multiple purposes in the industry. Among its many functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. The mud is mixed at the surface and pumped downhole through a bore of the drillstring to the drill bit where it exits through various nozzles and ports, lubricating and cooling the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore.
Furthermore, drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures thereby preventing fluids from blowing out if pressurized deposits in the formation are breeched. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e., the vertical distance from the surface to the bottom of the wellbore) and the density (or its inverse, specific gravity) of the fluid used. Various weighting and lubrication agents are mixed into the drilling mud to obtain the right mixture for the type and construction of the formation to be drilled. Increasing the amount of weighting agent solute dissolved in the mud base will generally create a heavier drilling mud. Drilling mud that is too light may not protect the formation from blow outs, and drilling mud that is too heavy may over invade the formation. Therefore, much time and consideration is spent to ensure the mud mixture is optimal. Because the mud evaluation and mixture process is time consuming and expensive, drillers and service companies prefer to reclaim the returned drilling mud and recycle it for continued use.
Another significant purpose of the drilling mud is to carry the cuttings away from the drill bit to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling fluid exiting the nozzles at the bit stir up and carry the solid particles of rock and formation to the surface within the annulus between the drillstring and the borehole. Therefore, the fluid exiting the borehole from the annulus is a slurry of formation cuttings in drilling mud, and the cutting particulates must be removed before the mud can be recycled.
One type of apparatus used to remove cuttings and other solid particulates from drilling mud is commonly referred to in the industry as a “shale shaker.” A shale shaker, also known as a vibratory separator, is a vibrating sieve-like table upon which returning used drilling mud is deposited and through which substantially cleaner drilling mud emerges. Typically, the shale shaker is an angled table with a generally perforated filter screen bottom. Returning drilling mud is deposited at the top of the shale shaker. As the drilling mud travels down the incline toward the lower end, the fluid falls through the perforations to a reservoir below thereby leaving the solid particulate material behind. The combination of the angle of inclination with the vibrating action of the shale shaker table enables the solid particles left behind to flow until they fall off the lower end of the shaker table. The above described apparatus is illustrative of one type of shale shaker known to those of ordinary skill in the art. In alternate shale shakers, the top edge of the shaker may be relatively closer to the ground than the lower end. In such shale shakers, the angle of inclination may require the movement of particulates in a generally upward direction. In still other shale shakers, the table may not be angled, thus the vibrating action of the shaker alone may enable particle/fluid separation. Regardless, table inclination and/or design variations of existing shale shakers should not be considered a limitation of the present disclosure.
Preferably, the amount of vibration and the angle of inclination of the shale shaker table are adjustable to accommodate various drilling mud flow rates and particulate percentages in the drilling mud. After the fluid passes through the perforated bottom of the shale shaker, it may either return to service in the borehole immediately, be stored for measurement and evaluation, or pass through an additional piece of equipment (e.g., a drying shaker, a centrifuge, or a smaller sized shale shaker) to remove smaller cuttings and/or particulate matter.
Screens used with shale shakers are typically emplaced in a generally horizontal fashion on a generally horizontal bed or support within a basket in the shaker. The screens themselves may be flat or nearly flat, corrugated, depressed, or contain raised surfaces. The basket in which the screens are mounted may be inclined towards a discharge end of the shale shaker. The shale shaker imparts a rapidly reciprocating motion to the basket and hence the screens. Material from which particles are to be separated is poured onto a back end of the vibrating screen, flowing toward the discharge end of the basket. Large particles that are unable to move through the screen remain on top of the screen and move toward the discharge end of the basket where they are collected. The smaller particles and fluid flow through the screen and collect in a bed, receptacle, or pan beneath the screen.
In some shale shakers a fine screen cloth is used with the vibrating screen. The screen may have two or more overlaying layers of screen cloth or mesh. Layers of cloth or mesh may be bonded together and placed over a support, supports, or a perforated or apertured plate. The frame of the vibrating screen is resiliently suspended or mounted upon a support and is caused to vibrate by a vibrating mechanism (e.g., an unbalanced weight on a rotating shaft connected to the frame). Each screen may be vibrated by vibratory equipment to create a flow of trapped solids on top surfaces of the screen for removal and disposal of solids. The fineness or coarseness of the mesh of a screen may vary depending upon mud flow rate and the size of the solids to be removed.
While there are numerous styles and sizes of filter screens, they generally follow similar design. Typically, filter screens include a perforated plate base upon which a wire mesh, or other perforated filter overlay, is positioned. The perforated plate base generally provides structural support and allows the passage of fluids therethrough, while the wire mesh overlay defines the largest solid particle capable of passing therethrough. While many perforated plate bases are generally flat or slightly curved in shape, it should be understood that perforated plate bases having a plurality of corrugated or pyramid-shaped channels extending thereacross may be used instead. In theory, the pyramid-shaped channels provide additional surface area for the fluid-solid separation process to take place, and act to guide solids along their length toward the end of the shale shaker from where they are disposed.
The filter screens used in shale shakers, through which the solids are separated from the drilling mud, wear out over time and need replacement. Because shale shakers are typically in continuous use, it is beneficial to minimize repair operations and their associated downtimes. Therefore, shale shaker filter screens are typically constructed to be quickly and easily removed and replaced.
There are currently several ways to secure screens to the shaker, including mechanical or pneumatic clamps, bolts, or wedge blocks that are hammered into place. For example, through the loosening of only a few bolts or the removal of a wedge block, the filter screen can be lifted out of the shaker assembly and replaced.
FIG. 1 illustrates attachment of a screen to a shale shaker 2. One or more shaker screens 4 may be installed in, or secured to, the shale shaker 2 with a wedge block 6. The screen 4 is placed on a support rail (not shown) and positioned underneath a stationary wedge guide 8. The wedge block 6 is then pounded into position so as to secure the screen 4 to the shaker separator 2. One of ordinary skill in the art will appreciate that the operator often chooses to use a combination of a hammer and a suitable piece of wood in contact with the wedge block 6 to deliver sufficient force to fully tighten the wedge block 6. As shown in FIG. 1, the wedge block 6 may also include a hammer surface 10 to aid in installation (as by pounding on surface 10a) and removal (as by pounding on surface 10b). Some prior art shale shakers have a hole-and-pin system to secure the position of the shaker screen 4 on the sealing surface of the shale shaker 2 during installation of the shaker screen 4 and tightening of the wedge block 6.
A similar basket and screen assembly is disclosed in U.S. Pat. No. 5,811,003, issued to Young, et al. The '003 patent discloses a separator screen installation system, including wedge blocks and vertical side rails. The screen frame rests upon support rails and the vertical side rails are positioned between the wedges and the screen. The wedges are hammered into engagement with a wedge angle, thereby applying a downward force on the side rails and the screen, securing them in place. The side rails may be fixed to the separator screen, and the side rails may be tapered down from the downstream end to the upstream end.
Due to the vibration or shaking of the screen separator, many parts in the separator may wear over time. Additionally, when using additional parts such as wedge blocks, the fine screen mesh may be easily damaged or ruined by accidentally dropping the wedge block or other parts onto the mesh. When the mesh is punctured in this manner on a new screen during installation, the screen must be replaced.
Accordingly, there exists a need for a screen frame that will reduce the downtime required to change screens. There also exists a need for a screen frame that will reduce the chance of damage to the screen during installation. It is also desired to minimize the number of parts that may wear due to the vibration and shaking of the screen separator.